Plenarvorträge - DPG-Tagungen

Magnetismus Donnerstag
MA 19 Hauptvortrag Tilgner
Zeit: Donnerstag 09:30–10:00 Raum: H10
Hauptvortrag MA 19.1 Do 09:30 H10
Magnetic field generation in planets and laboratory dynamos —
•Andreas Tilgner — Institut für Geophysik, Universität Göttingen,
Herzberger Landstr. 180, 37075 Göttingen
Most celestial bodies (the sun, most planets, stars and galaxies) generate
a large scale magnetic field by converting mechanical into magnetic
energy. In such a “dynamo process” electric currents are generated inside
a moving conductor which in turn create a magnetic field. In commercial
dynamos in use in cars or on bicycles, these currents flow inside especially
designed coils. In the earth however, the currents responsible for
the magnetic field flow in the liquid conductor (mostly iron) constitut-
MA 20 Magnetische dünne Schichten II
ing the core, which is spherical, devoid of electric structure and acts like
a massive short circuit. It was therefore disputed until the late 1950’s
whether the earth is capable of dynamo action. Doubts have been removed
on a theoretical basis and an experiment has demonstrated the
dynamo effect on a laboratory scale with a flow qualitatively resembling
the flow believed to exist in the core. Insights into the flow actually
realized in the core may be gained from numerical simulations, whereas
observations of the variations of the earth’s magnetic field allow to determine
the velocity at the boundary between the core and the electrically
insulating, solid mantle. We do not know yet what is driving the core
flow. The most probable mechanism is convection, but the precession of
the earth’s axis of rotation may also play a role.
Zeit: Donnerstag 10:15–13:00 Raum: H10
MA 20.1 Do 10:15 H10
Magnetic properties of cobalt thin films grown on Ni3Al(001)
— •Stella Maris Van Eek, Ioan Costina, Vitali Podgurski,
and René Franchy — Institut of Thin Films and Interfaces (ISG 3),
Forschungszentrum Juelich GmbH, D 52425 Juelich, Germany
The growth and the magnetic properties of thin cobalt films on
Ni3Al(001) at 300 K were investigated using Auger spectroscopy, low
energy electron diffraction, scanning tunneling microscopy and magneto
optic Kerr effect (MOKE). Hysteresis loops of the Co layer were seen
with MOKE in situ. The Co films were prepared with thicknesses from
1.8 ML to 8 ML and all were ferromagnetic at room temperature. The
coercivity of the Co film increased with deposited material up to 5 ML
and then decreased very slowly. We picked up a thickness of each one
of these two regions: 3 ML and 7 ML to investigate the magnetic properties.
At room temperature Co(001) grows fcc epitaxially due to the
small misfit between the lattice constant of fcc-Co and Ni3Al and forms
two-dimensional islands. Annealing up to 700 K did not show interdiffusion
but a coalescence and a flatening of the Co islands. The coercivity
field increased with annealing at 700 K, for example the coercivity of
the 7 ML Co sample showed an increase from 32(2) Oe to 43(2) Oe. For
the 3 and 7 ML Co samples the evolution of the magnetic properties
was investigated with the temperature. The magnetic ordering temperature
values for the different thicknesses are reduced compared to the
bulk one, this was explained with the scaling theory, which predicts a
weakened magnetic order at the surface.
MA 20.2 Do 10:30 H10
Absorptionsfeinstruktur an den L2,3 Kanten der leichten 3d
Übergangsmetalle — •A. Scherz, H. Wende, C. Sorg, K. Baberschke
und E.K.U. Gross — Fachbereich Physik, Freie Universität
Berlin, Arnimallee 14, 14195 Berlin-Dahlem, Germany
Eine systematische Studie des magnetischen Zirkulardichroismus (XM-
CD) leichter 3d Übergangsmetalle (TM’s) ist an Fe/TM/Fe(110) Dreilagen
durchgeführt worden. Im Gegensatz zu den schweren 3d TM’s weisen
die XMCD-Spektren eine Vielzahl von Absorptionsfeinstrukturen auf. Es
ist bekannt, daß Korrelationseffekte mit dem Rumpfloch die spektrale
Verteilung der L2,3 Kanten in den isotropen Absorptionsspektren (XAS)
beeinflussen. Dennoch besteht bislang keine umfassende theoretische Beschreibung
der Rumpflocheffekte in Absorptionsspektren entlang der 3d
Reihe für den Festkörper. Ein einfaches Modell wurde erarbeitet, welches
es ermöglicht, die Korrelationsenergien aus den experimentellen XAS-
Spektren zu bestimmen. In Verbindung mit der Analyse der XMCD-
Spektren kann direkt gezeigt werden, daß die gewohnte Anwendung der
Summenregeln zur Bestimmung von Spin- und Bahnmomenten, µS und
µL, aufgrund dieser Korrelationseffekte für die leichten 3d TM’s versagt.
Eine alternative Herangehensweise zur Bestimmung von µS und µL, die
die spektrale Linienform des XMCD analysiert, wird daher vorgeschlagen.
Die Ergebnisse der systematischen Studie erlauben einerseits eine
Abschätzung der magnetischen Momente aus experimentellen Daten als
auch andererseits eine Überprüfung zukünftiger Theoriemodelle, die die
Rumpflocheffekte in Absorptionsspektren von magnetischen Festkörpern
beschreiben. Gefördert durch das BMBF (05KS1 KEB4).
MA 20.3 Do 10:45 H10
MODIFICATION OF THE SPIN DENSITY WAVE OF CR
IN FE/CR MULTILAYERS BY INSERTION OF SN — •D.
Lott 1 , D. Solina 1 , M. Almokhtar 2 , K. Mibu 3 , W. Schmidt 4 ,
and A. Schreyer 1 — 1 GKSS Research Center, Geesthacht, Germany
— 2 Physics Department, Assiut University, Egypt — 3 Research Center
for Low Temperature and Materials Sciences, Kyoto University, Japan
— 4 Institut Laue Langevin, Grenoble, France
Recently, the SDW behavior of Cr in Fe/Cr/Sn/Cr multilayers were
studied by Moessbauer spectroscopy. The monolayer (ML) of Sn inserted
into the Cr layers serves as a Moessbauer probe permitting the
study of the magnetic local environment around Sn providing details
about of the magnetic stucture of Cr. In this work, complimentary
neutron diffraction studies were carried out on a set of Fe/Cr(t) and
Fe/Cr(t/2)/Sn(2˚A)/Cr(t/2) multilayers with t=80˚Aand t=160˚A. The
magnetic order of the Cr layers was examined, systematically comparing
the systems with and without Sn inserted in the Cr layers. The multilayers
with Sn show major changes with additional asymmetric modulations
in the diffraction spectra along (00L). Even more dramatic are the
changes for the Fe/Cr/Sn/Cr sample with t=80˚A. Instead of a CSDW
to ISDW transition with increasing T as seen in the sample without Sn,
a dominant CSDW component is observed at low T transforming into a
ISDW phase with higher T. Measurements and evaluations clearly show
that the general SDW behavior of the Cr layers in Fe/Cr multilayers are
changed drastically by insertion of Sn allowing to tailor the magnetic
properties of the Fe/Cr system in a new way.
MA 20.4 Do 11:00 H10
Temperature-dependent magnetotransport measurements of
epitaxial Fe/Cr/Fe systems with antiferromagnetic interlayer
coupling — •Matthias Buchmeier, Michael Breidbach,
Henning Dassow, Daniel E. Bürgler, and Peter Grünberg
— Institut für Festkörperforschung, Forschungszentrum Jülich GmbH,
52425 Jülich
We have prepared epitaxial Fe/Cr/Fe trilayers with antiferromagnetic
interlayer coupling on an Au(001)/GaAs(001) buffer-system. Lithographically
patterned thin stripes are characterized by current-in-plane magnetoresistance
(CIP-MR) measurements, for which the magnitude as well
as the in-plane direction of the external magnetic field are swept. The
temperature dependence (4–270 K) of the in-plane anisotropy, the interlayer
exchange coupling, and the giant and anisotropic magnetoresistance
(GMR and AMR) are quantitatively determined by simultaneously fitting
the angular and field dependence of the MR curves. The experimental
curves are nicely reproduced within a single domain model including fourfold
magnetocrystalline anisotropy and bilinear and biquadratic coupling
terms for temperatures above 50 K. Below 50 K, however, we observe an
anomaly which can be interpreted as an exchange bias (EB) effect with
an exchange anisotropy of the order of 5 mT. Possible origins for the
appearance of EB is a paramagnet-antiferromagnet phase transition of
the Cr spacer or an Fe-oxide layer on top of the uncapped sample with
a thickness of approximately 1 nm as determined by X-ray scattering.